1. Field of the Invention
The present invention relates in general to the telephone transmission of data allowing the phone subscriber to receive only calls from persons who are in possession of a pre-established code. The operations allowing this are performed at the exchange to which the phone set of the subscriber pertains. If no code is received at the exchange, this latter will not send the ringing signal to the subscriber's phone set.
2. Description of the Prior Art
A phone subscriber is commonly pestered by innumerable unwanted phone calls, even in the small hours or when he is busy and he does not desire to be perturbed. A method to obviate such inconveniences is to have a private exchange attended by an operator who acts like a screen between the calling and the called parties. However, it may happen that the operator is not sure of the importance of the call; in such case the operator must assume the responsibility of transferring the communication, in which case said operator must interrupt the called party or, in the opposite case, look for an excuse for obviating said interruption. In this latter case it may happen that the operator is wrong in evaluating the importance of the true urgency of the message motivating the call. A device, supposedly intended for obviating inconveniences of this sort, is the automatic answering recorder. However, this kind of apparatus does not allow the called party to know if the calling party is a desired one, before the latter hangs up. Such an apparatus is useful for knowing which persons have called during a period of absence of the phone subscriber. Later on he may replay all the recorded messages which have been left by the calling parties during his absence. It is not practical to put in operation such an apparatus when the phone subscriber is present because he may lose the chance to receive important or simple desired calls at the moment when they are being made. It is also possible to resort to a company rendering answering services. While such services offer obviously several advantages, they also have various inherent drawbacks. One of them is that the answering service retains the phone line of the phone subscriber during all the time the service is rendered. Another drawback is that the operators of such services become acquainted with the nature of the received message and thus there is no privacy or secrecy. Still another drawback lies in the fact that there is a considerable delay between the reception of the message by the answering service and the moment when the called subscriber becomes acquainted therewith. Another device used by some phone subscribers to get privacy, and which is available on the market, is a simple switch connected in series with the circuit of the ringing bell of the phone set. When the switch is open, the bell will not sound and thus the phone subscriber will never become aware of the fact that a phone call is being made to him. This is obviously a very inconvenient method, since the phone subscriber may miss one or more very important and/or urgent calls. The phone subscriber has no means to know if a call is being made and if the calling party is or is not a desired one. Another very important drawback is that if the phone subscriber forgets to close the switch when he desires to be again in condition of receiving calls, he may remain uncommunicated during several hours, or even days, without becoming aware of the fact.
Also a method and apparatus have been suggested which allow the phone subscriber to receive calls only from persons which are in possession of a pre-established code. However, it is necessary to provide such an apparatus at the same location at which the phone subscriber's set is located. However, according to this invention, similar functions may be performed by an apparatus located at the exchange to which the phone subscriber pertains. Notwithstanding, the means for carrying out such functions are in this case somewhat different, and some are differently arranged, with respect to the means suggested by the prior art method and apparatus.